1. Field of the Invention
The present invention relates to an image distribution apparatus preferable in using in an image distribution system that distributes images, a communication terminal apparatus, and a control method thereof, and more particularly relates to detection processing of changes in distributed images.
2. Description of the Related Art
In recent years, image distribution systems that continuously distribute images to client-side via a data transfer medium such as the Internet or an intranet and monitoring systems are already diffused, being used in various fields of, for example, distribution of live video images, indoor and outdoor monitoring, or observation of plants and animals. Further, some of these systems are provided with a function (a function of image change detection) that detects changes in images with time.
However, the conventional function of image change detection determined whether there has been a change in an image, according to the size of a pixel difference value between a current frame and a previous frame, which does not allow accurate determination on image changes due to effects by noise or the like. Further, it is not possible to set conditions of determination on image changes in the function of image change detection as conditions desired by users.
Still further, although the Internet is widely used as a user-friendly data transfer medium connecting image distribution apparatuses with communication terminals on client-side recently, a sufficient transfer capacity is not necessarily secured, because a transfer route is not fixed.
For example, in the case of using a modem, which is typically used with a personal computer, as a communication section with connection to a telephone line as a part of a transfer medium, the transfer rate is several ten k bit/second, and thus the transfer capacity is not large enough to continuously distribute images. A transfer capacity larger than several hundred M bit/sec is required in order to transfer present TV video signals with no degradation. Even using an efficient dynamic image coding system such as MPEG-2 (Motion Picture Coding Experts Group-2), which is an international standard, degradation is apparent with a data rate as low as 1 M bit/sec. Therefore, when transfer capacity is several ten k bit/sec at most, the image size is reduced, the frame rate is dropped, and the compression rate is increased, thereby reducing the data rate at a cost of degradation of the image quality of reproducing images.
Accordingly, in continuously distributing images via a low speed transfer medium, also due to an insufficient transfer capacity for transferring data other than image data (coded data), results of image change detection, which are transmitted from an image distribution apparatus to communication terminals on client-side, include only digital and simple information just for notifying as to whether there has been a change in an image for each frame.
Accordingly, communication terminals on client-side receive only very simple results of image change detection, making it impossible for clients to grasp optimum parameters for image change detection (parameters to decide a situation in which image changes can be determined) or to find optimum parameters that detect image changes.
Nor is it possible to set desirable parameters for image change detection, from client-side to the image distribution apparatus via communication lines.
As described above, image data is usually coded to be transmitted to a transfer medium. Depending on the transfer speed of the transfer medium, the transmitting frame rate of image data is in general smaller than the frame rate of coded image data. In other words, a part of coded image data is not transmitted but is disposed. On the other hand, detection of image change is performed on all frames of input image data, thus results of image change detection are transmitted from the image distribution apparatus to the communication terminals of client-side one-sidedly. Accordingly, image change detection is performed with a rate greater than the transmitting frame rate. In addition, image data and results of image change detection are not always distributed with synchronization, and thus it is difficult to correctly recognize the results of image change detection in detail, for example, as to which region of which image frame changed at which timing.
Therefore, in trying to set parameters of image change detection from client-side, for example, detection situation is not recognized completely in correspondence with frames, causing a problem of requiring try and error for several times before parameters are decided.